Example
5. Valid and true
All dolphins are mammal. (true) X is a dolphin.
Therefore, X is a mammal. (true)
6. Valid but false
All dolphins can fly. (false) X is a dolphin.
Therefore, X can fly. (false)
Example
7. Invalid but true
Water is not fire. (true)
Fire is not wind. (true)
Therefore, wind is not water. (true)
8. Invalid and false
Water is fire. (false)
Fire is wind. (false)
Therefore, wind is water. (false)
Remarks
If an argument is valid and each premise is true, the conclusion must be true.
But if there is a false premise, the conclusion must be false.
Terminology of argument evaluation
- Truth (true / false)
- Validity (valid / invalid)
- Soundness (sound / unsound) - Strength (strong / weak)
- Cogency (cogent / uncogent)
Term used in both deduction and induction
Truth: the detail of each statement is fact
(empirical data) which can be proven by experience e.g.
Bangkok is the capital of Thailand. (True)
All Thais are Europeans. (False)
Ghost does exist. (Undetermined but in reality it must be true or false.)
Terms used in deduction
Validity: the logical relation between the premise and the conclusion. It concerns about whether the conclusion can be derived from the
premises or not.
If the conclusion is derived from the premises (valid)
If the conclusion is not derived from the premises (invalid)
Soundness: the quality of an argument as being both valid and true. Only the case that the argument is valid and each statement is true, the argument is considered as a sound (good) argument.
Other cases are unsound (bad) argument.
Steps to evaluate a deductive argument
Sound
Deduc&ve Argument
True Valid
False Invalid
Unsound Unsound
Remarks: a sound argument is valid and each premise is true.
Terms used in induction
Strength: the numbers of evidences supporting the conclusion
If the conclusion is supported with sufficient evidences. (strong)
If the conclusion is not supported with sufficient evidences. (weak)
Cogency: the quality of an argument as being both strong and true. Only the case that the argument is strongly supported with sufficient evidences and each evidence is true, then the argument is considered as a cogent (good) argument. Other cases are uncogent (bad) argument.
Steps to evaluate an inductive argument
Cogent
Induc&ve Argument
Strong True
Weak False